Mean Sea Level (MSL) is the datum for measurement of elevation and altitude.
Mean Sea Level is the equipotential surface of the Earth as described by the WGS84 geoid
The gravity field of the earth is neither perfect nor uniform because density (and therefore mass) varies throughout the planet. This is due to magma distributions, mountain ranges, deep sea trenches, etc. If the Earth was a perfect sphere covered in water, the water would not be the same height everywhere. Instead, the water level would be higher or lower depending on the particular strength of gravity in that location.
The surface of the oceans that would exist if the oceans were in equilibrium - that is at rest, unaffected by wind, currents and moon and interconnected by canals across continental land masses - would be an equipotential surface.
A geoid is a mathematical model of the Earth, a smooth but irregular surface that corresponds not to the actual surface of the Earth's crust, but to an equipotential surface. A geoid is the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level.
The World Geodetic System is a standard for use in cartography, and navigation. It comprises a standard coordinate frame for the Earth, and a gravitational equipotential surface (the geoid) that defines the nominal sea level.
The latest revision is WGS 84 (dating from 1984 and last revised in 2004), which will be valid up to about 2010. WGS 84 is the reference coordinate system used by the Global Positioning System.